Final answer:
The ratio of snails to goldfish in Kelly's aquarium is 1:3.
Step-by-step explanation:
The ratio of snails to goldfish in Kelly's aquarium can be found by dividing the number of snails by the number of goldfish. In this case, there are 7 snails and 21 goldfish, so the ratio of snails to goldfish is 7/21. This fraction can also be simplified to 1/3. Therefore, the ratio of snails to goldfish is 1:3.